This invention relates to a distillate material having excellent suitability as a jet fuel with high lubricity or as a blending stock therefor, as well as the process for preparing the jet fuel. More particularly, this invention relates to a process for preparing jet fuel from a Fischer-Tropsch wax.
Clean distillates streams that contain no or nil sulfur, nitrogen, or aromatics, are, or will likely be in great demand as jet fuel or in blending jet fuel. Clean distillates having relatively high lubricity and stability are particularly valuable. Typical petroleum derived distillates are not clean, in that they typically contain significant amounts of sulfur, nitrogen, and aromatics. In addition, the severe hydrotreating needed to produce fuels of sufficient stability often results in a fuel with poor lubricity characteristics. These petroleum derived clean distillates produced through severe hydrotreating involve significantly greater expense than unhydrotreated fuels. Fuel lubricity, required for the efficient operation of the fuel delivery system, can be improved by the use of approved additive packages. The production of clean, high cetane number distillates from Fischer-Tropsch waxes has been discussed in the open literature, but the processes disclosed for preparing such distillates also leave the distillate lacking in one or more important properties, e.g., lubricity. The Fischer-Tropsch distillates disclosed, therefore, require blending with other less desirable stocks or the use of costly additives. These earlier schemes disclose hydrotreating the total Fischer-Tropsch product, including the entire 700xc2x0 F.xe2x88x92 fraction. This hydro-treating results in the complete elimination of oxygenates from the jet fuel.
By virtue of this present invention small amounts of oxygenates are retained, the resulting product having high lubricity. This product is useful as a jet fuel as such, or as a blending stock for preparing jet fuels from other lower grade material.
In accordance with this invention, a clean distillate useful as a jet fuel or as a jet fuel blend stock and having lubricity, as measured by the Ball on Cylinder (BOCLE) test, approximately equivalent to, or better than, the high lubricity reference fuel is produced, preferably from a Fischer-Tropsch wax and preferably derived from cobalt or ruthenium catalysts, by separating the waxy product into a heavier fraction and a lighter fraction; the nominal separation being, for example, at about 700xc2x0 F. Thus, the heavier fraction contains primarily 700xc2x0 F.+, and the lighter fraction contains primarily 700xc2x0 F.xe2x88x92.
The distillate is produced by further separating the lighter fraction into at least two other fractions: (i) one of which contains primary C7-12 alcohols and (ii) one of which does not contain such alcohols. The fraction (ii) is a 550xc2x0 F.+ fraction, preferably a 500xc2x0 F.+ fraction, more preferably a 475xc2x0 F.+ fraction, and still more preferably a n-C14+ fraction. At least a portion, preferably the whole of this heavier fraction (ii), is subjected to hydroconversion (e.g., hydroisomerization) in the presence of a bi-functional catalyst at typical hydroisomerization conditions. The hydroisomerization of this fraction may occur separately or in the same reaction zone as the hydroisomerization of the Fischer-Tropsch wax (i.e., the heavier 700xc2x0 F.+ fraction obtained from the Fischer-Tropsch reaction) preferably in the same zone. In any event, a portion of the, for example, 475xc2x0 F.+ material is converted to a lower boiling fraction, e.g., 475xc2x0 F.xe2x88x92 material. Subsequently, at least a portion and preferably all of the material compatible with jet freeze from hydroisomerization is combined with at least a portion and preferably all of the fraction (i) which is preferably a 250-475xc2x0 F. fraction, and is further preferably characterized by the absence of any hydroprocessing, e.g., hydroisomerization. The jet fuel or jet fuel blending component of this invention boils in the range of jet fuels and may contain hydrocarbon materials boiling above the jet fuel range to the extent that these additional materials are compatible with the jet freeze specification, i.e., xe2x88x9247xc2x0 C. or lower. The amount of these so-called compatible materials depends on the degree of conversion in the hydroisomerization zone, with more hydroisomerization leading to more of the compatible materials, i.e., more highly branched materials. Thus, the jet fuel range is nominally 250-550xc2x0 F., preferably 250-500xc2x0 F., more preferably 250-475xc2x0 F. and may include the compatible materials, and having the properties described below.
The jet material recovered from the fractionator has the properties shown in the following table:
The iso-paraffins are normally mono-methyl branched, and since the process utilizes Fischer-Tropsch wax, the product contains nil cyclic paraffins, e.g., no cyclohexane.
The oxygenates are contained essentially, e.g., xe2x89xa795% of oxygenates, in the lighter fraction, e.g., the 250-475xc2x0 F. fraction, and are primarily, e.g., xe2x89xa795%, terminal, linear alcohols of C6 to C12.